A physicochemical process for refining metal surfaces is described and claimed in Michaud et al U.S. Pat. No. 4.491,500, issued Jan. 1, 1985, which process involves the development, physical removal and continuous repair of a relatively soft coating on the surface. High points are leveled through mechanical action, preferably developed in vibratory mass finishing apparatus, and very smooth and refined surfaces are ultimately produced in relatively brief periods of time
The patentees teach that the process can be carried out using either a part-on-part technique or by incorporating an abrasive mass finishing media; e.g., quartz, granite, aluminum oxides, iron oxides, and silicon carbide, which may be held Within a matrix of porcelain, plastic, or the like. As described therein, the effectiveness of the process is evidently attributable to the selective removal of surfaces irregularities, Which removal has been facilitated by chemical conversion of the metal to a softer form.
To achieve ultimate refinement of the metal surface, it will generally be desirable to finish the Michaud et al process with a burnishing step, which may be carried out by treatment of the parts in a mass finishing unit charged with a so-called burnishing media and an aqueous alkaline soap solution, the latter being inert to the metal. Such burnishing media will typically be composed of mineral oxide grains fused to a hard, dense, non-abrasive cohesive mass; it is also commonly known to use steel balls for burnishing metal parts.
The process described by Michaud et al can be employed to produce burnished parts without transferring them to a second bowl, by using a relatively nonaggressive cutting medium (e.g., a ceramic containing 10 to 15 percent of abrasive grit). ln such a procedure the initial, surface-refinement phase is carried out with a reactive solution which produces the conversion coating on the parts, generally followed by a flushing step and then, with the equipment in operation, a flow of a burnishing soap solution.
Although highly advantageous, such a method may not produce specular brightness, since it is characteristic of abrasive media that they scratch the metal surfaces. In Michaud U.S. Pat. No. 4,818,333, issued Apr. 4, 1989, a physicochemical process is provided for refining metal surfaces to a condition of high smoothness and brightness, in relatively brief periods of time and without need for removal of the objects or the media from the container of the mass finishing unit. The process is characterized by the use of a non-abrasive, high-density burnishing media throughout the entire surface-refining and burnishing operation.
As indicated, the physicochemical refinement methods described in the foregoing patents involve the formation of a conversion coating on the metal surface, which is ultimately removed in the burnishing step. Because that occurs primarily through physical contact, however, some of the coating frequently remains in sheltered or recessed areas. This is of course undesirable for self-evident reasons, especially if the part is to electroplated, varnished, or otherwise surface coated.
At present, hydrochloric acid is widely used to dissolve such residual conversion coatings, but that practice is undesirable for a number of reasons, particularly the tendency of HCl to cause hydrogen embrittlement. Other chemical formulations have been employed for the dissolution of oxalate and phophate coatings, but they are typically characterized by relatively high levels of organic component content; thus, they disadvantageously add to the oxygen demand made upon available waste treatment facilities, and in some cases their use is prohibited as a result.
The Prior art of course discloses numerous chemical formulations for cleaning metal surfaces, many of which employ a phosphate compound as the primary active ingredient. For example, Crowther U.S. Pat. No. 2,986,526, issued May 30, 1961, discloses metal-cleaning compositions which comprise an alkali metal pyrophosphate and a higher aliphatic fatty alcohol/ethylene oxide reaction product; tetrasodium pyrophosphate is deemed to provide the best result, and is preferred. In accordance with the patent, a granular product is made by absorbing the ethylene oxide/-alcohol adduct into the pyrophosphate, at ratios in the range of 0.5-10:90-99.5, and the product is dissolved in water at a concentration of 0.5 to 10 percent and to provide a working solution with a pH of preferably 9.0 to 10.0.
Copson U.S. Pat. No. 3,325,244, issued June 13, 1967, and Van Kampen et al U.S. Pat. No. 3,370,015, issued Feb. 20, 1968, disclose cleaning compositions in Which a pyrophosphate is the major ingredient. Cinamon, Kelly et al and Sopp, Jr. disclose, in U.S. Pat. Nos. 2,481,977, 3,210,278 and 3,655,467, issued Sept. 13, 1949, Oct. 5, 1965 and Apr. 11, 1972, respectively, compositions containing a pyrophosphate and another alkaline detergent builder. Phosphate cleaning compositions are also taught by Schaeffer, Highfill, and Dupre et al, in U.S. Pat. Nos. 2,618,604, 4,803,058, 3,145,178 and 3,312,624, issued Nov. 18, 1952, Feb. 7, 1989, Aug. 18, 1964, and Apr. 4, 1967, respectively.
Despite the activity in the art indicated by the foregoing, a need remains for a burnishing composition and mass finishing method by which metal-surfaced objects can be refined using a physicochemical technique, and can subsequently be burnished while simultaneously effecting the removal of any residual conversion coating. It is therefore the broad object of the present invention to provide a novel composition and method for achieving those results.
It is a more specific object of the invention to provide a novel composition and method by which such residual coatings can readily be removed from areas of metal surfaces that are recessed, or are for other reasons inaccessible to contact by solid elements employed in a mass finishing process
Other more specific objects are to provide such a composition and method by which the workpiece surfaces can be brought to a condition of specular brightness, in a desirably brief period of time and without etching or other adverse effect upon surface quality.
An especially important object of the invention is to provide a composition and method having the foregoing features and advantages, which produces a waste stream having a low chemical oxygen demand characteristic, and which is relatively easy to treat for the recovery of dissolved metal compounds.
A further object of the invention is to provide a novel burnishing composition composed of ingredients that are readily available and relatively inexpensive, which can be prepared in stable, concentrated form so as to make transport and use convenient and economical.